Electric coupling circuits



FBI). 9, 1932. WHEELER 1,844,374

ELECTRIC COUPLING CIRCUITS Filed Aug. 20, 1930 INVENTOR Harold A.Wheeler ATTORNEY5 Patented Feb, 9, 1932 PATENT OFFIC lElIlllFtlULD A.WHEELER, JAUKSUN HEIGHTS, NEW YORK, ASSIGNOR TO HAZEL'I'INE UORPOMTIUN,A UORPOJRA'IION OF DELAWARE ELECTRIC CUUPLINU- CIRCUITS Applicationtiled. August $0,193). Serial No. 4%,541.

'lhis'invention relates to hi h fre uenc electric coupling circuitstunable over 'a range in frequency and adapted more especially forll'llfilCOllllCClllhg successive elements of a high frequency thermionicamplifier or lTl manner which is under the control of the designer; thedesign in general being preferably such as to produce a substantiallyconstant amplification, i. e. uniform sensitivity throughout the tunablefrequency range.

With an elementary type of high frequency coupling circuit, of which atwo-winding transformer having a tunable secondary is a typical example,the variation in amplificaiil tion with frequency is not under thecontrol ill the designer, but increases with frequency throughout thetunable range, in a manner well understood, so that the resultingsensitivity is widely different at the lower and higher frequencies.

in the elementary type of coupling circuit referred to-above, thereexists between the primary and tunable secondary circuits, a degree ofcoupling which is substantially constant throughout the tunable range, acondition which is partially responsible for the mentioned increase inamplification with frequency. The present invention avoids the resultantdisadvantages of the elementary ent invention both as regards thecircuit connections utilized and the specific modes of operationinvolved, are set forth in Patent No. 1,763,380 issued to C. E. Trube,and in his co-pending applications Serial No.

101,906, filed April 14, 1926 and Serial No.

120.045, filed July 2, 1926.

The coupling circuits of the present invention comprise the followingessential elements, namely: a tunable secondary circuitincluding a fixedinductive winding and variable capacity bridged between a pair of outputterminals, and a primary circuit including in closed series connectionin the order named, a portion or all of the secondary winding, a fixedcapacity, a fixed inductance, and a primary winding magnetically coupledto the secondary winding and of relatively small magnitude compared tothe fixed inductance. The primary winding is so poled as to givecoupling from the primary to the secondary circuit in additive relationto the coupling to the portion of the secondary winding which isincluded in the primary circuit. lnput terminals are connected to theprimary circuit between the fixed capacity and inductance and betweenthe primary and secondary windings, respectively, so that there isprovided a pair of parallel paths, one path containing the fixedcapacity and the portion of said secondary winding included in theprimary circuit, while theiother path contains the fixed inductance andthe primary winding magnetically. coupled to the secondary. i

An additional capacity may or may not be connected directly between theinput terminalS accOrding to whether or not there is i already presentsuilicient capacity for the purpose explained below. On the other hand,sufficient capacity may exist between these terminals due to the type ofinput conne'cted thereto. In any event, the resulting capacitynssociatedwith the inductance of the primary circuit, including the fixed capacity of the closed series circuit which is connected in shunt with theprimary circuit, as well as any capacity existing between-the inputterminals, should be such as to render the primary circuit resonant at afrequency lower but not greatly lower than the lowest frequency withinthe tunable range.

The objects-attained by such design (1) provide an effective couplingbetween primary and secondary circuits which automatically falls ofi asthe frequency of tunin is increased, ,at a rate proportioned to profucea desired variation in amplification with frequency, and (2)" controlthe variationsin input impedance of the coupling circuit throughout thetunable ran e in such manner as to insure the desired per ormance. j

The manner. in which the results of the present invention are attainedwill be .more

clearly understood upon detailed consideration of thecircuit connectionsset forth in the drawihgs, of which:

tubes in cascade relation; while Figure 5 shows an interstage connectionemploying triode tubes wherein the coupling circuit is arranged inconjunction with a suitable capacity to provide neutralizatiqnof theinterelectrode tube capacity.

Like elements are similarly designated in the several figures.

:Referring to the drawings, the coupling circuit of-.Fig. 1 comprises aresonant secondary circuit consisting of a fixed inductive winding L anda variable tuning condenser G bridged between output terminals, and apri mary. circuit associated with the input terminals I including inclosed series connection in the ordennamed, the entire winding L, a

fixed capacity C ,ja fixed inductance L and a primary winding L,magnetically coupled.

to the secondary winding L as indicated, and of relatively few turnscompared tofthe fixed inductance L The winding L is so poled asto givecoupling from the primary to the secondary circuit in additive relationwith the couplin to the winding L.

A pair- 0 input terminals I .are connected I between'the fixed capacityC and theinductance L and between the primary winding L and thesecondary windingL respectively,

thereby providing the parallel paths between the input terminals one ofwhich contains the fixed capacity C and secondary winding L,

while the other path contains the fixed inductance L and the primarywinding L The capacity C shown dotted is intended to represent theentire capacity connected between the lnput terminals duringoperationofthe circuit, including any capacity 'efiects due to the inputimpedanceconnected thereto together with the capacity of'a supplementarycondenser which may be connectedbetween these terminals. I

i The capacities C and G together with any additional capacity efiectsassociated with the primary circuit inductance which includes theelements L L and L, should be such as to render the primary circuitresonant at a frequency lower, but not greatly lower, than the lowestfrequency in the tunable range as determined by adjustment ofcondenserC.

Itis not essential to the operation of the circuit that a capacity (Lexist between the input terminals I. In the event that the capacity C iszero, however, itis required that the capacity C be of such magnitude asto render the primary circuit resonant at the low frequency abovespecified.

The reactances of the primary circuit are so proportioned that the pathbetween the input terminals containing the elements 0 and L is caacitatively reactive throu h-. out the tuna le range, and. hence ofecreasing impedance for increases in the tuned frequency; whereas thepath containing elements L and L being of necessity inductivelyreactive, the impedance thereof increases with the tuning frequency.Thus, the windings L and L produce efiects in the secondary circuitwhich vary in opposite fashion with the tuning adjustment, the resultanteffect being proportioned to produce an efi'ective coupling betweenprimary and secondary circuits which automatically decreases withincreasing tuning frequency, in .a manner' which with suflicientaccuracyis expressed by the following equation:

4 ime oar where is the resonant frequency of the primary circuit,'f'thefrequency of tuning, while m and m are constants expressed by InEquations (2) and (3) the symbols L, C

and C have the significance shown in the drawings; while M is the mutualinductance between windings L and L. It will be observed from Equation(1) that; the efl'ective coupling must necessarily decrease withincrease in the tuned frequency.

' The modification of Fig. 2 differs from Fig. 1 in that condenser C isconnected to an intermediate point of winding L so that only a portionL; of the secondary winding L is included in the primary circuit.

The operation of the circuit oat Fig. 2 is substantially identical withthat of Fig. 1. The coupling constants m and m however, are changed fromthose given in Equations {2) and (3) to the following expressions:

where ltl is the mutual inductance between windings L and lb, thesignificance ot the remaining symbols being the same as for Equations(2) and in Fig. 3 the input terminals T of the coupling circuit cit Fig.2 are connected to an antenna 1 and a ground 2, respectively, while theoutput terminals are connected to the grid and cathode, respectively, ofa thermionic tube "t1. The capacity C: of Figs. 1 and 2 comprises inFig. 3 the effective antenna-togronnd capacity or the antenna 1 which inconjunction with capacity C renders the primary circuit inductancecomprising elements li la and L, resonant at a frequency below thetuning range.

Tn T ig. lwherein the coupling circuit interconnects a pair otscreen-grid tubes, V and in cascade relation, the input terminals it areconnected to the anode and cathode respectively of tube T while theoutput terminals U are connected between grid and cathode respectivelyof tube V it. battery B tor supplying space current to tube W} isserially interposed in the shunt path containing elements L and Ti, withthe negative battery terminal joined to the grounded conductor 3Q Thecapacity (5 of Fig. 2 comprises in Fig. l. the anode-to-ground capacityot tube T7 which together with capacity in renders the inductance of theprimary cir- I cuit resonant at the mentioned low frequency.

The coupling circuit of l ig. 5 interconnects a pair ct triode tubes incascade rela tion, each tube having anode, cathode and grid electrodesonly. Tn order to neutralize grid-to-anode capacity of tube V aeondenser G,, is connected from the anode thereof to the point betweenthe fixed inductance T1 and the primary winding T1 this point being atopposite polarity to that of the upper terminal o't winding L to whichthe interelecrode capacity Q is connected. The neutraliaation isaccomplished in the manner disclosed in Patent No. l,lfi9,22fi issued toL. A. lt-laaeltine, April l, 1924.

Tn rug. 5 the capacity C comprises the anode-to-ground ot the tube V,plus the capacity ot the associated neutralizing condenser Un which isetlectively in parallel therewith, owing to the small inductance of thewinding L While the coupling circuit of Fig. 2 is shown in all of thevarious modifications-in Figs. 3 to 5, inclusive, it is to be understoodthat the coupling circuit of Fig. l'might in each instance have beenequally well employed, additional figures including such disclosurebeing deemed unnecessary.

It will be noted that in Figs. 3 to 5 inclu sive the circuit arrangementissuch that the rotor element of the tuning condenser may be groundeddirectly, constituting a very desirable feature in that it eliminatescertain incidental couplings between the several parts of the amplifier.

ll claim:

1. An electric coupling system comprising a tunable secondary circuitincluding a fixed secondary winding and variable capacity bridgedbetween output terminals, and a primary circuit including in closedseries connection in the order recited, at least a portion of saidsecondary winding, a fixed capacity, a fixed inductance, and a primaryWinding magnetically coupled to said secondary and of relatively fewturns compared to said fixed inductance, an input terminal connectedbetween said fixed capacity and inductance, and a second input terminalextending between said primary and secondary windings, the inductanceincluded in said closed series circuit together with the associatedcapacities including any capacity bridged between said input terminalsin the operation of said circuit, being resonant at a frequency slightlybelow the tunable range.

2. An electric coupling system comprising a tunable secondary circuitincluding a fixed secondary winding and variable capacity bridgedbetween output terminals, and a primary circuit including in closedseries connection in the order recited, at least a portion 0t saidsecondary winding, a fixed capacity, a, fixed inductance and a primarywinding of relatively few turns compared to said fixed inductance andmagnetically coupled to said secondary winding, an input terminalconnected between said fixed ca pacity and said fixed inductance, and asecond input terminal connected between said pri mary and secondarywindings, and a fixed capacity bridged between said input terminals,said fixed inductance together with the associated capacities beingresonant at a frequency slightly below the tunable range "forcontrolling throughout said frequency range the coupling between saidprimary and secondary circuits. I

3. An electric coupling system comprising a tunable secondary circuitincluding a fixed secondary winding and variable capacity bridgedbetween output terminals, and a' primary circuit including in closedseries connection in the order recited, at least a portion of saidsecondary winding, :1 fixed capacity, a fixed inductance, and a primarycompared to said fixed inductance, an input.

' terminal connected between said fixed capacity and said fixedinductance, a second input terminal connected betweensaid primary andsecondary windings, said fixed inductance together with theassociatedcapacities including any capacity bridged between said input terminalsin the operation of said circuit, being resonant at a frequency slightlybelow the tunable range, said primary and secondary windings havingopposite polarities.

4. In a radio receiving system the combination of ahigh frequencyelectric coupling circuit comprising a tunable secondary circuitincluding a fixed secondary winding and variable capacityf bridgedbetween grid and cathode of a thermionic tube; and a primary circuitincluding in closedseries con-' nection in the order recited, at least aportion of said secondary winding, a fixed capacity, a fixed inductanceand a'prilnary winding magnetically coupled to. said secondary winding,an input terminal connected between said fixed capacity and said fixed.in-

, ductance, a second input terminal connected between said primaryand-secondary windings the terminals of which thus joined are ofopposite polarity, and an antenna circuit connected to said inputterminals, the inductance of said primary circuit together with theassociated capacities includingthe capacity of said antenna effectivelybridged between said input terminals, being resonant at a frequencybelow the tunable range, for automatically controlling throughout saidrange the coupling between said primary and secondary circuits.

5. A high frequency thermionic amplifier comprising the combination ofan electric coupling circuit having a tunable secondary circuitconsisting of a fixed secondary winding and variable capacity bridgedbetween grid and cathode of a first thermionic tube, a primary circuitincluding in closed series connection in the order recited, at least aportion of said secondary winding,-a fixed capacity, a fixed inductance,and a primary winding magnetically coupled to said secondary a secondthermionic tube having an anode connected between said fixed capacityand inductance,-and a cathode connected between said primary andsecondary windings the terminals of which thus joined are of oppositepolarity, the inductance of said primary circuit together with theassociated capacities including the anode-'to-ground capacity of saidsecond tube being resonant ata frequency slightly below the tunablerange, for providing automatically throughout vsaid range uniformly highdegrees of amplification. I

6. A high frequency'thermionic amplifier comprising the combination ofan electric circuit including a fixed secondary winding and variablecapacity bridged between grid and cathode of a first-thermionic tube, apri coupling circuit having a tunable secondary mary circuit includingin closed series connection in the order recited, at least aportion ofsaid secondary windlng, a fixe'd capacity, a fixed inductance, and aprimary winding windings the terminals of which thus joined are ofopposite polarity, and a condenser magnetically coupled to saidsecondary 7 proportioned, to neutralize the anode-to-grid capacity ofsaid first tube connected from the anode thereof to a point between saidprimary winding and said fixed inductance, said fixed inductancetogether with the associated capacities including the anode-to-groundcapacity of said second tube and said condens-- ers being resonant at afrequency below the tunable range, for providing automaticallythroughout said range .uniformly high degrees ofamplification in theoperation of said amplifier.

In testimony whereof I affix my signature.

HAROLD A. WHEELER.

